This invention relates to an electric power steering apparatus in which part of a steering force is provided by an electric motor.
A known apparatus of this general type is disclosed in Japanese Laid-Open Patent Application No. 227560/1984 and also in FIG. 20 of the drawings.
In this apparatus, a steering rotational shaft 102 is axially displaced by a plurality of steel ball bearings 103 into and out of a ball nut 101 positioned in a steering gear box 100 so as to slide horizontally as viewed in FIG. 20. When the shaft 102 is rotated, the steel ball bearings 103 move along a spiral groove formed on the outer surface of the shaft 102 so as to thereby move ball nut 101 axially.
A rack 101a is formed on and extends axially along the outer periphery of ball nut 101 and meshes with a pinion 104a of a sector shaft 104. Accordingly, when ball nut 101 moves axially, as described above, the sector shaft 104 is rotated to thereby move the wheels via a steering member, not shown, connected to sector shaft 104.
The steering rotational shaft 102 is positioned coaxially with a steering shaft 105 integral with a steering wheel, not shown. Both shaft members 102 and 105 are substantially hollow and fixedly connected to each other via a torsion bar 106 inserted therethrough at opposite ends of the torsion bar. The end of shaft 102 which is connected to torsion bar 106 is connected by a reduction device 107 to a drive shaft of an electric motor 108. A torque sensor 109 is provided at the end of steering shaft 105 opposite to the steering wheel. The output signal S1 sensed by torque sensor 109 represents the direction and torque of rotation (hereinafter referred to as torque) of the steering wheel is input to a controller 112 along with the output signal S2 from a vehicle speed sensor 114. An output drive signal S3 from controller 112 controls the direction and torque of rotation of electric motor 108.
As described, the electric power steering apparatus of this known type is intended to provide an arrangement in which the electric motor shares the steering force input in accordance with the direction and torque applied to the steering shaft 105 as sensed by torque sensor 109.
In this known steering apparatus, the direction and torque of the steering shaft are sensed by a single torque sensor to drive the electric motor, so that if trouble occurs in the torque sensing unit, a signal will possibly be produced irrespective of the driver's intention, thereby driving the electric motor and automatically steering the vehicle and possibly leading to an accident. In addition, the torque sensor is operated by use of a relative deviation between the input shaft and the output shaft. Accordingly, if an abnormality occurs in the operating mechanism, a smooth feeling of steering will not be obtained. In a worst case scenario, if the operating mechanism is seized or becomes inoperative during the course of the steering operation, a signal will continue to be produced regardless of the driver's intention and an electric current will continue to flow to the electric motor causing an automatic input to the steering thereby leading possibly to an accident.
It is a technical subject of this invention to prevent a signal of the foregoing type from being generated when there is abnormality in the torque sensor. This would avoid driving the electric motor and reduce the risk of accidents.